Impact line printers comprise an endless type carrier, such as a band or belt with etched characters, revolving at constant speed past a row of electrically operable print hammers. An electronic control system selectively actuates the print hammers in synchronism with the motion of the type carrier to impact a print medium against selected characters as they come into alignment with selected print hammers. The quality of printing depends on accurately timing the hammer impact with the alignment of the selected character at the selected hammer position.
The type band commonly has timing marks that physically locate each character on the band. They also have a reference or "home" mark that identifies which character timing mark is the start of the band. The timing mark following the home mark is the home character timing mark. Sensors read the timing and home marks. A single sensor may be used if the home mark is embedded in the timing mark track. Due to space constraints associated with the hammer unit assembly, the band drive mechanism and the paper feed carriage mechanism, the sensors are not usually located at the home hammer position. Therefore, the home character, i.e. the character physically located by the home timing mark, is displaced from the home timing mark by the distance between the home hammer and the timing mark sensor. This distance varies from printer to printer by the accumulated tolerances derived in the manufacturing processes of the hammer unit, the band drive mechanism, the band and the sensors.
The timing mark sensors, including the home mark sensor if separate, must be adjusted until the correct characters are printing in the correct hammer positions. This is done by adjusting the sensed home pulse until it accurately indicates when the home character is correctly positioned to be struck by the home hammer (true home). True home cannot be calculated without actually knowing where the print hammer is striking the band. Sensed home is adjusted by adding a delay equal to the difference between sense home and true home (home pulse delay). One way the adjustment can be made is by having a fixed home pulse delay and mechanically moving the sensor until home pulse delay is correct. U.S. Pat. No. 3,987,723 describes a sensor adjustment mechanism. Other mechanisms may be seen on commercially available printers such as the IBM 3262, 4245 and 4248 line printers.
Another way for adjusting the sensed home is by printing a test pattern and visually observing the characters printed. If the test pattern looks as if it is off center, the home pulse delay is changed until the test pattern looks correct by manipulating an input device such as an operator panel which inserts a delay value adjustment into the control electronics of the system. U.S. Pat. No. 4,368,666 shows another method of adjusting print hammer timing which involves manual adjustment of circuit parameters of the hammer operating circuits to compensate for the interval from the detection of the timing mark and the energization of the hammer electromagnet.
A problem with the mechanical adjustment method is its increased cost factor. In addition, it is time consuming and the accuracy achieved is dependent on the manual and visual skills of an operator to produce the right setting of the mark sensor. The problem with the test pattern method is that it is to large extent subjective and requires manipulation of an operator panel.